When I went to see Hamilton a few weeks ago, I came out thinking: wow, by far and away the best actor on that stage was the one who played Aaron Burr, third vice-president of the United States and killer of the leading man, rendered into a complex R&B anti-hero by Lin-Manuel Miranda’s script – kind of Milton’s Satan meets R Kelly.

Who was that actor, and where did he come from? I asked myself. I tore through the programme – but saw a different face entirely under the name of Burr. The man I had seen – the best man in the show – was the understudy.

His name is Sifiso Mazibuko, “principal standby”. He is from South Africa – and he is not only required to play Aaron Burr if the other Burr gets sick: he is understudy for Alexander Hamilton, too. And for Thomas Jefferson. Oh, and Revolutionary War general the Marquis de Lafayette as well. He must deliver any one of these four principal roles with a few hours’ notice. They sing 41 songs between them, and the songs mostly feature two or more of the parts. “I am expected to know my stuff,” says Mazibuko.

At any point during a rehearsal of Hamilton you will see, around the auditorium, a clutch of “standbys” mirroring the movements from the stage, moving like shadows in the aisles, “trying not to get in the way, but trying to get as close to the action as possible”. The key to learning four big parts at once is in the muscle memory, says Mazibuko. And in colour-coding, too. Hamilton is green, he decided. Jefferson and Lafayette are purple. And Burr is orange – “because he is the story, the narrative, the whole theatre with that lush amber lighting”. Mazibuko’s wife, also an actor, helped him with the script, rapping the other parts. He learned 80 per cent of each role, starting with his favourite, then “fed in” the remaining information when he could almost make a decent fist of each. For songs such as “Non Stop”, where Burr and Hamilton perform in a kind of theatrical split-screen effect, he rapped one part out loud, while doing the other in his head.

One night in December, Mazibuko got a call from the company manager warning him, “You may have to be on tomorrow, I will confirm in the morning.” He hoots with laughter at the thought of the night that followed. In the morning it was confirmed. He told the manager, “I would maybe like to come in a little earlier to the theatre, and do some work on the part before the show, if that’s OK?” His wife packed his lunch.

Mazibuko comes from Kimberley in the Northern Cape and speaks five languages. He was originally going to study law until he heard about a musical theatre course at Pretoria University. His parents advised him to try it out. After a master’s in acting at Ohio State, he and his wife came to London, where the size of the burgers made more sense to them. He played Marvin Gaye in the West End.

Hamilton’s associate choreographer, Stephanie Klemons, told Mazibuko that in some mysterious way, the actors who play Burr are always similar. “They keep to themselves,” he says. “Burr waits for stuff. Doesn’t go full on. A guy on the outside looking in, saying, ‘No, that’s not how I do things’.”

The decision to shoulder the mighty part of quadruple understudy, which balances the luxury of not having to perform every night with the pressure of having to do so at short notice, was strategic. If he could play one role, just once, it would be worth it. He adds: “I was thinking, how do I get to be part of that room?” The character of Burr, political outsider, has permeated him. His signature song is “The Room Where it Happens”.

He hasn’t played Hamilton, Jefferson or Lafayette yet, but he’s done Burr 30 times. Every other theatre job will be simple now. The principal actor for Burr, Giles Terera, has just won an Olivier Award. But those of us who’ve seen the other one – whose pop-up appearances are, of course, entirely unpredictable – all assumed he was the star. On the average day, he comes to the theatre anyway, and is to be found up in the dressing room, watching TV, “eating food” and putting himself through his paces.