Grandma's House, which stars Amstell as a version of himself, focuses on a television presenter looking for something more meaningful to do with his life.
In an interview with TV Choice, Amstell explained that he found acting "really difficult".
"I was really terrified, after spending so much time trying to make the scripts as authentic, heartfelt and real as possible, that I might go in and screw the whole thing up with my horrific acting," he said. "But everyone was very kind and generous to me."
He revealed that he met "various acting teachers" and attended a clown course recently, adding that he often asked the rest of the cast for advice.
However, he admitted that he is unsure whether he will return to acting in the future.
"Let's see how it goes, see what the people say," he said. "It's not up to me, really. People could say, 'This is awful, you can't act' and I could carry on regardless but it would seem a bit insane. Also, I don't know if I'm professional enough to be able to read somebody else's lines. I couldn't even get my own right!"
Grandma's House begins on Monday at 10pm on BBC Two.