If I have a nickname I always go by and that everyone knows me as / calls me, when do I tell this information to a potential employer? Most applications do not have a spot on them to write in a nickname or alternate name, and it would seem awkward to just randomly blurt out "I go by..." when you first meet them. So, when is the best time and how do I go about revealing that information? Do I tell them at my first interview, whenever they first use my name, or when they've offered me the job?
If your "nickname" is what EVERYONE, even strangers, calls you (for example it says Katherine on my birth certificate but I go by Kate) just do all your interactions with the company using that name (fill out the application form if there is one, sign your cover letter and so on.) Then where necessary mention your other name - for example that a university degree was earned under that more formal name. I occasionally have to ask a client who is buying a plane ticket to buy it for Katherine Gregory so the ticket will match the passport.
If your "nickname" is what your friends call you, and you don't expect strangers, even those who work in the same company, to use it, then wait until you are hired and invite people to call you it if you like. If you want all your coworkers to use it, wait for the first person to either greet you, or introduce you to someone else, and say "I go by animuson actually" and then that person will introduce you to everyone that way. If during the interview process you're more comfortable being called by the nickname, but for some reason you applied as LongName, when it's your turn the person will say "good morning, you're LongName?" and you'll say "yes, though in person I prefer to go by animuson" and they will make a note on their notes and you're all set.
It really depends on if an opportunity presents itself. I have a name that is commonly "nicked" (Jacob --> Jake). Many interviewers will ask my if I prefer one or the other, at which point I can make my preference known. If an opportunity such as that does not present itself, I would communicate it to HR after accepting an offer and then to my new boss on my first day (with reinforcement as I'm being introduced around if necessary.)
This is simply a matter of professionalism. If your nick-name is acceptable within the culture of the company, there should be no reason not to introduce yourself with that moniker during the interview.
If however your nick-name is more personal than a contraction or common substitution (Dave for David, Bob for Robert) then you might want to wait until you have the job before going around and telling people to call you something else.
Essentially, at an interview, I would expect to call someone by the name the provided on their CV (resume), but I would be happy to accept that people may prefer to use a different name for official correspondence or a contract than in personal interactions.
You have a couple of opportunities. Some put their nickname on their resume, others (Michael to Mike) mention it in the interview. You can also wait until you put together their email signature.
The confusing ones are those that can make it hard to find the employee in the email system. You may want to let your company know before IT has a chance to set up the email system.