Do you need an alternate?
 

I am prepping for an interview and need a little help on the question " Do you need an alternate?" I understand the 1-2-3 rule (121.619) , my question is what about 121.621 (wx must be 1500' above circling mins etc.) When do you use that? Or does that regulation only apply to flag and 135 operations? Thanks all!

If you're operating under 121 domestic regs, you have to comply with the 1-2-3 rule under FAR 121.619.

For 121 flag operations and all IFR 135 operations, you comply with FAR 121.621/135.223.

Now, keep in mind that even 121 domestic uses the derived minimums for weather AT the alternate airport, as specified in OpSpec C055 - this applies to all 121 and 135 operations under that OpSpec.

Ok so 121.619 is for domestic, and 121.621 is for flag 121 and all 135........correct?

Basically, yeah. Technically 121.621 is only for flag 121, while 135 is governed by 135.223, but the actual weather requirements of the rules are the same.

Ok thanks a lot!!

Think globally when answering these types of questions. The intent is not only to determine your technical knowledge of the regs, but also your "SA" (Situational Awareness)... It would not be a bad idea to bring up the fact that both the PIC and Dispatcher need to jointly concur on the wx, fuel, alternates, etc.. Use CRM and get everyone involved in the process that should be. The interviewers will love you for it!

The simple answer is...you always need an alternate. The 1.2.3. rule (or variance of that rule) is the exception, or how you get around carrying the extra fuel for the alternate.
Other than that, the other answers are good.