My interview is scheduled at 12:00. In my interview invitation the interviewer asks me to call him when I arrive, so he can pick me up at the reception. This is a rather big company where he might have to walk 5-10 minutes to get me. Due to the train connection I will be there about 45 minutes early.
Should I call him at 12:00 sharp or ~10 min earlier so he can pick me up at 12:00?
I am pretty sure when the interview is scheduled at 12:00 that it means to be in the building at 12:00 and not in his office as you (I suppose) have no been there before, so I would not take his walking time in consideration.
But I would call him at 11:55 because it is neither too early nor too late. It shows that you can uphold appointments and even leave a small amount of room for possible distractions, and that you can manage your time to be there at the RIGHT time and not half an hour too early.
I believe 5 minutes early is the sweetspot to call him.
I usually aim to arrive for interviews 15-30 minutes before the stated time; in your case 11.30-11.45. I check in with Reception who usually notify the interviewer I've arrived. This allows plenty of time to wait behind other people, sort out a toilet visit if needed, to sit back and relax prior to the oncoming grilling and to have a look at some of the company literature to help pad out what I've already learnt about the company. Occasionally I'll be picked up before the appointed time but this is unusual.
If you turn up at 12.00 you're effectively already late, which suggests poor planning, which is not a good first impression to give.