Jim Parsons and Mayim Bialik had "some nerves" about appearing in the 'Young Sheldon' finale.

The 'Big Bang Theory' spin-off came to an end this week, with the two stars reprising their roles as adult Sheldon Cooper and his wife Amy Farrah Fowler, and though the pair have provided voiceovers for the prequel, it was still daunting to step back into their old roles.

Executive producer and co-showrunner Steve Holland told People magazine: “They’re both so talented. I think they would say that there was some nerves about stepping back into these characters and that they weren't quite sure they would find their rhythm. From watching them do it, it felt so effortless.

"Just watching them work, sitting, editing and watching the takes, they make so many smart and interesting choices on each line. It was just really a joy to watch."

But Steve admitted it wasn't difficult to persuade Jim - who was also an executive producer on 'Young Sheldon' - and Mayim to be part of the finale.

He said: “I think they were both excited. Jim's obviously been a part of the show. He's an executive producer. He's been the narrator.

"I think Mayim literally said, ‘Anything you want me to do, I'll come there and do.’"

Producers had to navigate Jim's schedule, as he is currently starring in a Broadway production of 'Mother Play - A Play in Five Evictions' and ended up having to fly to Los Angeles on a day off from rehearsals before returning to New York for his commitments to the play the following day.

Their scenes were also shot earlier than the rest of the finale to accommodate the pair's busy schedules.

The show followed Sheldon Cooper as a nine-year-old prodigy with his family in Texas as he navigated high school and the wider world.

CBS Entertainment's Amy Reisenbach recently hailed the comedy as proof "lightning can strike twice" after it looked to recapture the huge success of 'The Big Bang Theory'.

She added: “As a prequel to one of the biggest comedies, Young Sheldon proved lightning can strike twice.

"It set itself apart with a remarkable cast that felt like a family from the first moment we saw them on screen and brought characters to life with unique heartfelt stories that drew audiences in from the start."