They can also, on occasion, misfire spectacularly.
And while Say When definitely leans towards the former, something about it is just well...
Let's start at the start.
We have Keira Knightley playing a 20-something someone (Megan). A someone who knows not who she is or what she wants.
She's managed to drift through life quite brilliantly, to the extent she's late to her friend's hen party because she was busy waving a "tax office" sign outside her father's business.
WHO USES A WAVED SIGN TO GENERATE BUSINESS IN THE TAX WORLD?????
I mean come on, who the hell drives along, sees that sign, and suddenly thinks "shit, of course, I don't understand my tax returns!"?
But I digress.
Megan is still friends with her old schoolfriends - with whom she has little in common - and is still with her High School boyfriend Anthony (played by Mark Webber).
While drifting along, events conspire to make Megan wonder where the hell her life is going.
Her solution is to lie to everyone and run away. So far, the most sensible thing she's done with her life.
Running away leads her to the house of Annika (played brilliantly by Chloe Grace Moretz), a shoolgirl she befriended while running away from her best friend's wedding.
So naturally, she ends up staying with the schoolgirl - and her dad - while pretending to everyone else that's she's at some personal growth seminar.
At this point you, as the viewer, have a decision to make.
Either get on board with the conceit that a 20-something would enjoy hanging out with a 15/16 year old, or get off the bus.
I'm not saying it couldn't happen, but it raises more questions than it answers.
And this is a film that raises a lot of questions, right from the off. When you find yourself asking, two minutes in, who the hell took the swimming pool picture then, you know you're in for an interesting time.
But anyway, where was I?
Oh yes, lying to all and bunking with her new friend.
From here, we're into the realms of personal journey shrama, as only an American indie comedy can do.
Megan makes mistakes and learns stuff. Annika makes mistakes and learns about other people's mistakes right on cue (well not quite, I was banking on counting to three before she appeared in one scene, and it was on 2 and a half).
Children learn from adults and vice versa and all is good.
I know I'm sounding dismissive at this point, and I don't mean to be. The film is what it is, and does it perfectly well.
Although you can only call it a comedy during the scenes with Sam Rockwell.
Playing the divorced divorce lawyer and dad to Annika, the film comes alive when he's on screen.
It's not obvious, the pace doesn't change, but there's a noticeable shift from the film staring at its shoes to looking up and smiling.
And that's the main problem with this film.
While well made and fairly well written, it kind of feels like a bolognese with a key ingredient missing.
Yes, it's still recognisable as what it's meant to be, but something's not quite right.
It's not the editing, which is a bit overly harsh at times, it's not Lynn Shelton's direction (even if the odd shot raises eyebrows), it just feels a little flat.
And it's not even the fact that the final scene made me almost shout at the screen (a personal thing I know, but I was really hoping for a different ending).
I suspect part of the problem can be laid at the doors of focus groups (which would also explain why it's called Laggies in America, and we get Say When - a title which actually makes less sense).
It feels too safe, too sanitised. A film that could have had more on an edge but lost the courage of it's convictions.
To be fair to Knightley, she does well. OK, the American accent jars slightly to these ears, but overall she convinces as a shallow woman who hasn't bothered to actually mature and grow up.
She's outshone by Rockwell, sure, and CGM (as her friends probably call her) too for that matter, but I can't think of a film Rockwell hasn't stolen.
And CGM is custom-made for this stuff.
No, the actors are not at fault. Far from it. It's just this is a potentially filling dish crying out for some spice.
See, Laggies. Nope, no idea.