Hmm. Ok, the "very subtle" bit I entirely understand. Does it state (or did they state) that it has to be a "monologue
from a film"? Or was it a "monologue suitable for film" ? There's a big difference, and the second one is much easier!
I'm going to assume it's the second, because I honestly can't think of any British films with
monologues suitable for 14 year olds. There must be some, but - honestly - I can't think of any at all!
I would assume that what the agency wants (because this will be common to most of them) is to see what you would be like as a screen actor. Most of the professional work you are likely to be put up for by an agent if you are aged 14 will be commercial, TV or film (I doubt that there will be very much straight theatre available to someone of your age) - so it's mainly your suitability for screen work they want to assess. Most amateur actors have much more experience of stage, so they are presumably trying to guide you towards the sort of performance in the agency audition that they want to see.
So -
Don't worry whether what you choose comes from a film or not.
Choose something (anything!) that is written in the sort of language and tone you usually use. Do you have any novels at home that have teenage characters in? Sometimes you can adapt dialogue from a novel and cut and paste bits together to make a monologue. The same is true of plays written for schools - you can often cut and paste a few short speeches together to form a longer one (this is entirely acceptable for audition purposes). You could look at some monologue books for teenagers and see whether you can find something there.
You might like to look for a piece that is quite introspective and thoughtful. This can come across well for screen acting. It allows you to take time over your thoughts and if the agent can see you truthfully reacting to your thoughts and thinking as you speak, this is probably the closest you can get to the skills needed for screen performance when doing a monologue.
To be honest, the most important thing is how you deliver it, rather than precisely what it is. The "age appropriate" thing is very important, because it will be hard to make it believable otherwise. It's also important that you can make them believe that it really could be you saying it (so, no Shakespeare or Dickens, because you presumably don't speak in the way that they wrote!). If you can find something modern and conversational it will allow you to put all your effort into making it utterly truthful in performance. Watch plenty of TV drama before your audition and see how "careless" or even "hesitant" the speech is in some of it. You don't need to speak "beautifully", you just need to
be the person you've chosen in whatever situation they are in. - You probably know all that!
I don't know whether any of that is helpful - I do hope it is. Best of luck!
