Habitually, fortuitously so.
At the moment, the actor is uneasy about his career choice: "Do grown-ups say, 'Yes, the thing I most want to do is put on a frock and mince around pretending to be someone else'? I don't know."
It was adolescent unease that got him into the profession and proprietorial unease that nudged him into a role where he was crowned a certain sort of a king: Mr Darcy.
It's the emotion he excruciatingly manifests in his latest project, a movie about the distress of an adult son facing his overbearing, imperfect father's death.
"It's very rare that you see something which is so unsentimentally frank," Firth says of the allure of When Did You Last See Your Father?
"And I think I was just grateful to see that."
Firth first encountered that frankness in Blake Morrison's memoir of the same title, years before there was talk of turning it into a movie.
"It was one of those books which seems to come every 10 years for me, which stands out and you want to evangelise to everybody," he says.
Through strained bedside interactions and flashbacks to childhood, Firth's character wrestles with his outsize image of his father, the secrets that formed fissures between them and his standing in the ailing man's eyes.
"I just don't trust anything that proposes solutions," says the actor, known for his mastery of the brooding malcontent.
"Any positive place [the movie] gets to has been hard-earned by facing conflicts."
The 47-year-old Brit decided he wanted to act at age 14, at least partly out of contempt for academia.
"I just had this incredible sense of liberation from schoolwork and everything else I was doing - being bad at maths and chemistry," he recalls.
"Suddenly I had this alibi: 'I don't need this; I'm going to be an actor'."
And he was, quickly landing stage and film roles after drama school.
More than a decade into his career, a producer sent him the script for a six-part television adaptation of Jane Austen's Pride and Prejudice.
"I had no interest whatsoever," Firth recalls.
With the support of his advisers, he at first turned the role of Mr Darcy down.
"Everybody at my agency [said], 'Don't do it. It's a step down.' There was one camp saying, 'I've been in love with Mr Darcy all my life. Don't do it.'
And there was another camp saying, 'The part is unplayable. He belongs in literature'."
Literature that Firth had never even read before the part was offered.
But he did read it, and while the surly aristocrat grew on him, he continued to say no.
Until one morning, he says, he woke up with that familiar unease and "suddenly decided it would feel very odd watching someone else do it".
The mini-series debuted in 1995; Darcy has been fixed to Firth ever since.
"I've run out of things to say about it," he says of his fictional shadow.
"I've even gone beyond answering questions about how it feels to be answering the questions."
Firth has played into the joke, appearing as strait-laced Mark Darcy in Bridget Jones's Diary, and then tried to shake it. (He's not exactly a proper gent in 2005's Where the Truth Lies.)
Even as he has made peace with the Darcy factor, the actor perpetually finds himself asked to play to a type he doesn't much like.
"A guy whose basic job is to ache over a girl is not the most interesting thing for an actor to be faced with," he says.
"Romantic by itself is insipid. I think the reason why Darcy works, if he works, is his first job is not to be romantic . . . He's a misanthropic [jerk] really."
In the past two years, Firth has made something like 10 movies, including Helen Hunt's Then She Found Me, the musical Mamma Mia! and Easy Virtue, a comedy based on the Noel Coward play to be released next year.
"I'm getting this James Brown, hardest-working-man-in-the-business-thing at the moment," Firth says.
In fact, he adds, "I have been doing nothing - certainly nothing related to the business - since February . . . That does not make me feel overworked."
Which is as he wanted it.
The father of three says he has been itching to take a break from acting and spend some time with his family.
"In some ways I feel like I'm stuck with a decision I made when I was 18, being an actor. I don't think it's the decision I'd take today."
So today he's uneasy.
Tomorrow? Perhaps the next brilliant turn.
When Did You Last See Your Father? opens in theatres next month.
