How will my boss react if he finds out I've been interviewing elsewhere?

Try to find out why and what do i want to not feel the need to search somewhere else (depends on the employee).

I did it back in 2001, and my employer found out and then they H.R. manager fired me.

She will tell me "good luck with that".

I cannot speak for others. In my case, my boss would be interested in why, but would otherwise be supportive.

It depends on the manager. A more emotionally mature manager who values honest, candid conversations might ask for feedback about your experience and why you're seeking to leave. They may also attempt to address some of your issues to encourage you to stay.

A boss will either take it as a personal insult due to ego or not care at all, since all employees are replaceable.

They should think about offering incentives to keep a good employee, though often they decide to let them go, or worse fire them.

He will start looking for your replacement immediately.

It should be a poor reflection on your boss, not you.

How will he find out? Are you talking about it with your peers?? That "should not be" his bizness.

It depends on the person. Some bosses (good ones) understand they have to keep their employees challenged and if they don't or can't employees will leave. Bosses who understand this aspect are usually ok with employees interviewwing till the time employee has been honest with them (let them know your career aspirations, challenges you are looking for and remiond them you are not being provided these opportunities). On the other hand bad booses (majority of them in my view) are not ok and will react harshly (knee-jerk reactions). So unless one is confident of their bosses' personality, tread with extreme caution.

It depends are you a great employee or a bad one he might be happy for you or he might get really upset that you are making them look for anew employee

Mine would probably let me go and leave yet another unfilled spot for the remaining staff to take on.

As a Manager, I would obviously be concerned. Instead of asking directly about the interviews, I'd ask questions to figure out why they would leave. This way I can see if I can resolve it without further ruining the relationship by confronting it. If I confront finding out, I'd expect the employee relationship to deteriorate further.

they will get jealous

Either supportive or indifferent, mainly due to the state of the organization and upper management that we work for, and also the negative conditions of the department that we work in.