Iker Casillas cannot remember what he was drawing at the exact moment he was called up to Real Madrid’s Champions League squad for the very first time but he knows he was drawing something. It was a technical design class, after all. One winter afternoon south of Madrid, a messenger came into his classroom to tell the teacher the headmaster wanted a word. Iker dashed to the head’s office where he was told he had to get home quick. Madrid were going to Norway and Casillas was going with them.

It was November 1997 and he was 16. He didn’t play, but a career had begun. Much has changed since then; much has happened too. It is the eve of Real Madrid versus Liverpool at the Bernabéu and the goalkeeper sits on the first floor of the club’s Valdebebas training ground. On Tuesday evening he will become the player with the most appearances in the tournament’s history and he will do so as the European Cup-winning captain.

He has won three of them. He has also won two European Championships and the World Cup as Spain’s captain. He says he thinks he has won 20 trophies and his calculation is correct. He has come a long way; the sport has changed and so has Spain. So too have Madrid; Casillas is a one-club man but the club is not the same. He has had 15 coaches and five presidents and it’s not just that he has gone from travelling to work by metro to travelling by car; the destination is different too: he’s even had three different training grounds.

The past two years may just have been the hardest of his 17-year career and the nerves never go entirely. The kid they called Saint Iker, performing miracles daily, became a devil in some eyes. José Mourinho dropped him and signed Diego López, publicly and aggressively expressing his dislike of Casillas. The goalkeeper admitted recently in a cautious if occasionally confessional TV interview that at times he felt “alone”.

He is not talking about that time explicitly now but it is hard not to see echoes of that when he discusses pressure and confidence, insisting: “I think a big part of success in football is mental, not physical. How you are inside your head matters more.”

After Mourinho, Carlo Ancelotti arrived and rotated the two; Casillas played in the Copa del Rey and the European Cup, López in the league. A bitter divide opened among some fans and the media, and not all the wounds have been fully healed. He is careful not to say so but when Casillas was welcomed with applause at Anfield he could have been forgiven for noting the difference.

The summer had felt like a watershed, or perhaps an opportunity to end it. A line could be drawn under his time at Madrid having finally won the décima and the talk was of a changing of the guard with Spain. He admitted that his World Cup had been a disappointing one. And while Casillas stayed, the start of this season was difficult too, although the team’s astonishing recent form has brought peace as well as goals and Ancelotti has kept faith with him, in the face of pressure.

“As a keeper you have more time and you do hear more from the stands,” he says. “There have been uncertain moments because of everything that has happened but I always think about the affection and the support that people have given me. I understand that some people might think differently but my duty is to take confidence from playing well. And I’m convinced that with my attitude and ability everything will go back to being the way it was before. That’s what we all want. The Madrid fans and me.”

Besides, Casillas insists he is not going anywhere and never was. He says he will play until he is 40, and there is a renewed ease about him as he takes a seat on the eve of a second game with Liverpool, and returns to that school room. “That was 1997 and now it’s 2014, so that’s, what, 17 years ago? Bloody hell,” he says. “I’m quite nostalgic. I like looking back over the papers and watching videos. And football’s nothing like it was then.

“Bodo Illgner was injured and [Santiago] Cañizares had a knock, so they needed me as a third keeper. They literally pulled me out of a technical design class. You can imagine the teacher saying: ‘Well he’s supposed to be working in my class.’ The headmaster, who was a big Madrid fan, called me. He knew I was in the youth system and every time I saw him he’d talk about Madrid. He told me that I had to get to the team hotel near Barajas, ready to fly to Norway.

“They took me home in the school minibus. My mum was frantically packing my bag for me. I got changed, picked up a suit and a jacket which I had from playing for Spain’s Under-16s and off we went. The other kids had no idea why I’d left class; no one had mobiles back then. Now everywhere you go everyone is waiting with a camera phone.”

So Casillas, a kid who was supposed to be at school drawing triangles, boarded a plane with the team that would finish that season as European Champions: a team that included Roberto Carlos, Clarence Seedorf and Davor Suker. Casillas didn’t know any of them. “Well, I knew all of them,” he says, “but they had no idea who I was. I sat at a table with Fernando Sanz, [Fernando] Morientes and Cañizares. I hardly said a word. I just sat there in awe. It was as if I’d won the lottery.”

Casillas knew he wouldn’t play; it would take a “domino rally” of injuries for him to get on. One was possible, as he would find out at Hampden Park in 2000, but two were not going to happen. Jupp Heynckes made him change and sit on the bench anyway for the experience. And something had been started. Two and a half years later, he was the youngest goalkeeper to play in a European Cup final as Madrid became champions against Valencia. And two years after that he came on against Bayer Leverkusen in Glasgow, saving his team in the dying minutes.

Ever since he had swapped gravel pitches for grass, Casillas preferred to play in short sleeves and the proof that he had not expected to come on was that he was wearing a long-sleeved goalkeeper top. His entry was delayed as he took a pair of scissors to it. Then came the saves and the trophy, a second European Cup at 20.

Afterwards, he cried with the tension of it all. His mother was not there; she had not expected him to play either. “I’m not sure it was an injustice but it felt like an injustice to me,” he admits. “The last two months of the season were the only months when I didn’t play. But you have to accept it, learn fast and mature, to be strong.” It would be a decade before those lessons needed to applied again. And it would be longer still before Real Madrid won the European Cup again.

“When you have won two European Cups at 20, it’s true that you do wonder if you’ll end up with five or six. It seems easy, but it isn’t. What’s happened since 2000 has shown that. We didn’t even get back to a final until this year - 12 years on. People stop appreciating how difficult it is if you win it every year. It can be hard to motivate people.”

A semi-final exit followed, then a quarter-final exit, then for six successive years Madrid fell at the first knockout phase. Under Mourinho they reached three consecutive semi-finals but the final resisted. Until last May, in Lisbon, Madrid finally won the European Cup with a 4-1 victory over Atlético Madrid – an experience Casillas described as better than wining the World Cup.

“If someone asks me after a final like that, when your adrenaline is through the roof, after you’ve been there for 90 minutes, wondering if you can come back and then Ramos gets the goal in the last minute, what am I going to say?” he protests now.

For Casillas, it had been especially emotional. Madrid had waited 12 years for the décima and Casillas had waited with them, the only player to do so. Having waited so long to get there, back in the team after a year of ostracism and a season of semi-ostracism, left out in the league, for a long time in Lisbon it looked like it might be his mistake that cost them the trophy. An untimely dash from the line had allowed Diego Godín to give Atlético a lead that lasted three minutes into additional time.

“I wanted the ground to open up and swallow me,” Casillas says. “The game was more or less under control but in an absurd play they scored. You spend the whole time with it going round and round and round your head. I’m looking round the stadium and can see thousands of people with white shirts supporting the team and I felt terrible that they weren’t going to get the European Cup.

“Sergio Ramos’s [equalising] goal meant lots of things and one of them was that he had rescued me. I’m sure I’d have got criticism every day. I’m sure quite a few papers had to change their headlines. Of course you’re conscious of that. I’ve been in the game a long time.

“I remember one [Gareth] Bale chance that went wide and you could see how nervous the fans were. You could see them thinking: ‘It’s coming, it’s coming, it’s coming’, but it never did. You’re on the goal-line thinking about it and you want to leave your penalty area to try to get the goal yourself.

“So you can imagine how I felt when Sergio’s header went in. The relief was total. I wasn’t just thinking about me, I was thinking about all the people there. It was huge.”

A few months later it was almost as if it if was forgotten. The summer sales of Xabi Alonso and Ángel di María drew criticism and Casillas was still under pressure, yet to fully recover his form. In part, that goes with the territory, he says. “Madrid is so big it seems like winning the European Cup buys you a month. When you come to this club you need a crash-course in impatience. There’s no time here.”

The early doubts have gone now; Madrid have won 11 in a row during an astonishing run of form that includes convincing victories over Barcelona and Liverpool. Trailing by six points after four games, they are now top domestically while victory on Tuesday would see them through to the next round with two games to spare. But Casillas isn’t taking that for granted. “English clubs,” he says, screwing up his face. “They’re never easy. They never back down and Liverpool need to win so it will be a great game.”

There is admiration as he talks about their opponents and about Steven Gerrard in particular. Casillas is a collector – “I’ve got [Gheorghe] Hagi’s shirt, [Lothar] Matthäus’s and Julio Salinas’s”, he says – and he got the Liverpool captain’s shirt after the first leg. This time, then, he will have a different target. “Seeing as I have Gerrard’s maybe I’ll ask for [Mario] Balotelli.”

Not at half-time, presumably?

Casillas laughs. “No, no, not at half-time. But anyway, the culture is different in Spain. You can swap shirts at half-time and it’s no problem.

“Players like Gerrard, Puyol, Totti, Xavi, have a different essence,” he continues. “They give the club its identity, a genuine link to the supporters.”

Before the first leg, Gerrard admitted that he had two opportunities to head to Madrid and eventually chose to stay. So has Casillas had the chance to go?

“There’s always speculation here but after all the stories, the rumours, I’m still here,” he says. “There’s never been a chance for me to go anywhere else because I’ve never thought about it. All the presidents I’ve had have said: ‘The day you don’t want to be here, just come and tell me.’ And I’m still here. It started here and I want it to finish here.

“I’ll play for Spain until 2016 at least, for sure. After that, we’ll see. And [for Madrid] I think I can play until I’m 40. I feel fit and strong. A keeper who looks after himself can play to at least 38. [Belgium’s Michel] Preud’homme was as fast as lightning and played until he was 38 at Benfica. Cañizares played until he was 38, [Oliver] Khan 39. And Gigi [Buffon] is 37 [in January].

“I saw [Faryd] Mondragón recently. He played at 43. If your head and body are fine, you can do it. And if you’re happy, if you have got a target, you never have a reason to go anywhere else. I’m happy. We waited a decade for the 10th. Now we’re thinking about the 11th. And we’ve got the team for that, so why not?”