With some bands, it doesn't even need to be done well because the fans will go see it anyway — and yet, to be done well it should really tell us things we didn't already know.
Ray, for example, and Get On Up shone a light into the darker corners of the lives of Ray Charles and James Brown respectively, elevating already good films and giving the audience something extra.
Which makes a biopic of Queen a tricky thing to tackle.
Already a production wracked with rumour, counter rumour, casting changes and director issues, it tells the story of a band and lead singer that's been covered many, many times since Freddie Mercury's passing.
Freddie's lifestyle, health, sexuality, ego — they are all the stuff of both fact and legend.
And if you're a true fan you already know that Smile became Queen (we only knew because we were at school with two huge devotees of the band), and things were never the same again. For anyone.
So how do you bring something new to this? How do you tell a tale already told?
...this is Queen...
...so you make the whole thing massively overblown and ludicrous and with a tongue so firmly planted in a cheek it's almost bursting through.
It helps, obviously, that your soundtrack is going to feature some of the finest songs ever committed to vinyl .
I mean, how can you go wrong when you are blasting Somebody To Love and Killer Queen at the audience?
Of course, as with all films, casting is key and in Rami Malek (yes, him from the Night At The Museum films) the producers found a guy who could not only live in Freddie's skin but bring the old dear to life in fantastic, glorious style.
Fittingly, he steals the whole show to the point you almost forget there are other members of the band.
As for the story itself, you have to remember this is a film and not a documentary because if you know your Queen history you'll find yourself tutting occasionally.
The fact this is a project brought to life by the two remaining active members of the band (Brian May and Roger Taylor if you've missed the memos) should allow you care slightly less about the liberties taken.
Should. But might not.
References to "but we haven't played together in ages" irk if you remember there was an album and tour the year before, and "from now on, all songs will be written by Queen, we'll share everything equally" sounds warm and fuzzy as long as you don't look at the writing credits for A Kind Of Magic.
Then there's the small matter of Freddie's partying and sexuality, one of which is kind of brushed to one side while the other is treated with kid gloves.
But such things are really just nitpicking.
While Freddie was often the story, Queen were still a band and they made their name on the road, and the live concert scenes really do bring a lump to the throat and make you want to be standing rather than sitting.
Of course, the whole thing is building to the Live Aid show that has long been credited with landing the band in the stadiums they were to play for the rest of their days with Mr M.
Bookending the film, and having watched the whole thing on the day, we were genuinely caught out with how emotional the closing 20 minutes of the film were.
And because we know now what we didn't know then, the show takes on an extra weight that, by the time the credits roll, has lifted you onto cloud nine and left you there.
We make no apologies for the sniffing and eye-dabbing that was going on over on our side of the screening.
Perhaps the most fitting thing about this film is how it starts slowly, builds gradually, and by the time the band hit their stride you're hooked and loving it.
Did we need Mike Myers in there? No. Did we need that joke about that song? No.
But when you find yourself hoovering up the band's back catalogue and the film's soundtrack in the days that follow, you know they got something right.
It's a far from from perfect film, but it grabs you by the heart strings and doesn't let go, and you feel like Freddie has popped back for one final encore.
Now, excuse me, my remastered copy of A Kind Of Magic has just turned up...