There's an RA job I want to apply for, and it's with a researcher I've been a subject for a few times, so we've chatted about the experiments he's doing, my experiment/masters/what's next etc. and he's talked about what I could do career-wise (also ran into him in the pub at a science-themed event I was helping with and we chatted some more).

I feel (and a careers advisor also thought) that it might be polite to drop him a quick email letting him know I'm applying for the job, but I'm worried that it might come across as looking like I'm somehow asking for special consideration. How can I phrase it/what can I say to make it clear that I'm just letting him know, as it's someone I do already know a bit, and not asking for anything?

Or am I being over-sensitive/should I just apply and don't need to let him know?

Definitely send him an email! But, I wouldn't frame it in a "i'm just letting you know I'm applying" tone.

Could you email him to ask a question about the post? Perhaps link it back into the experiments you've taken part in? That tells him that you're applying without looking like you're asking for special consideration. It's quite typical for prospective employees to ask questions about the post. Just make sure the question you ask isn't something you can find out easily.

For example, with a recent teaching post I applied for I emailed in advance asking if a start date of September would be considered (the funding for the post was available from July). It just meant I'd told my colleague I was applying, without explicitly emailing him to say "oi, remember me? I WANT THAT JOB!"

Thanks! Did wonder about that but then got stuck on what to ask...only thing I could think of is about starting date and seems more like something I should say if I actually got the job! But will have a think - good point re tying into experiments! Actually beginning to get a glimmer of a good question now...