Playing Jack and Karen – Will & Grace's hilarious pals in the hit sit-com – might sound like a barrel of laughs but as Sean Hayes and Megan Mullally tell us, there's plenty of pain and tears on set.

"Jack is a very physical character," says Hayes, who plays Will's flamboyantly camp best friend. "Here take a look at this," he says, rolling up his trouser leg and pointing to several cuts and grazes.

"This one is from a month ago, this from two months and this three months. I am constantly bumping into the furniture. Most recently I grazed my shin after jumping onto the table.

Supplied Will & Grace have returned to screens with a splash.

"It's a lot tougher doing such things now that I'm older but I make sure I do lots of stretches before filming to warm up and I really like the challenge of delivering."

Fans of the show know that no one enters or exits an apartment with Jack's flair, but coming up with different ways to do it can be taxing.

"These days the writers just type on the script, 'Jack Enters'," says Hayes, "and I go through different ways of trying to open the door! But that's the fun part.

NBC Sean Hayes as Jack McFarland and Megan Mullally as Karen Walker became the stand out stars of Will & Grace.

"Jack is so crazy that it's almost like there are no rules. He can almost do anything and get away with it. It's really hard to create such a character that people respond to positively. But it works with him and I love that."

Will & Grace was Hayes' first TV role. Now, as well as being one of the most loved sit-com characters, he has his own successful TV production company. It was announced last year he was helping to bring an American remake of the Kiwi comedy Step Dave.

"I'd auditioned for many pilots and done something like 50 or 60 commercials but that all changed in 1998 – the biggest year of my life, says the 47-year-old. "I was starring in two commercials that went out during the break in Superbowl, and that's a big achievement in America.

Sean Hayes/Twitter The cast of Will and Grace made a comeback in 2017.

"I had also landed a film called Billy's Hollywood Screen Kiss and I was at the Sundance Film Festival in Utah to promote it. I watched my adverts go out during Superbowl on the TV in my hotel room and felt very proud. Later, an executive from NBC who had watched a screening of the movie said he was casting for a new sit-com called Will & Grace and said, 'Would you ever come in and read for it?'

"I said 'Yeah' and two weeks later I got the role. It was like I had won the lottery. Of course, I had no idea that it would still be running today. I remember that the production company bought us all Porsches for being so successful so quickly and I sold mine in a day because I was like, 'I'm an out of work actor. Don't buy me a Porsche. Write me a cheque. I've got to pay my rent!' It was nice of them but I needed the money and I didn't want to pay the car insurance either. So I was being sensible."

Megan Mullally, who plays acerbic, motor-mouth Karen, loved getting the chance to show another side to her hard-faced character in this new series, particularly in the episode that sees the death and funeral of her cleaner, Rosario.

"That, for me, was the greatest episode every written for the character of Karen," she says. "It was important to me because I haven't been given very many opportunities in my career to show a different side of myself that's not just broad comedy. When they handed me that script I was overwhelmed by it and I wanted to rise to the level of the writing. I didn't want to let them down.

"There were two things in operation at once. There was the show in which the character of Karen loses who is essentially her best friend, and there was real life where the show lost the actress Shelly Morrison who has played Rosario so brilliantly for eight seasons because she decided she was going to retire and focus on her family and her husband. That added a great deal of poignancy for me. I felt genuinely moved."

In real-life, Mullally, 59, is the polar opposite of Karen and that is why she likes playing her so much.

"The writers very cleverly have Karen as the Donald Trump supporter, but she unwittingly has the best lines against him because the things she thinks are great about him are the most horrible to everyone else.

"I look nothing like Karen in real life and so was rarely recognised in public. It's only since social media and the fact that I post selfies on Instagram that people have started to recognise me.

"My natural speaking voice is very laconic and not the right pace for the show so over the years I made Karen's voice higher and faster so that dogs come running.

"My hair is naturally long and I wear little if any make-up day to day. I wear wigs for the role and once I have the pouf on top of my head and my Nancy Reagan finery, I feel like I am Karen. I call it getting into my drag!"