I gave a practice job talk to some individuals in my department yesterday and there were mixed comments about getting interrupted during your talk. At my graduate university, no one would have dared interrupted a job talk — but several individuals said this was common at their universities. I'm just curious now….
I don't know exactly what you mean by "interruptions" but I think it would be reasonable for someone to ask questions as the talk went along. In fact, I'd take it as a sign that the audience was interested and engaged. I suspect most will wait until after the talk, but I'd be ready to roll with questions or comments if they came up.
There is variability but I think it is becoming the norm for there to be questions during the talk. In fact, i would go into the job talk expecting this. I gave my job talk in 6 places and received questions in all of them - even the ones who claimed they usually waited till the end.
If you are getting clarification questions during your job talk at each campus visit, wouldn't that be a sign that you are not explaining things clearly enough? I would say getting questions of clarification during a talk is not a good sign.
I don't see it as a sign that you are not clear enough. Sometime a question is just a question. You are talking to a new and presumably very diverse audience that include both students and professors. You are also talking about a subject that most people may not know much about. It is almost impossible to clarify every aspect of your research in one talk. I actually think that if you do get questions, then it is a sign that you are doing well because the audience is attentive.
I agree that questions do not mean you are being unclear. Sometimes people have idiosyncratic concerns (e.g. they are trying to formulate an objection and want to know if you ruled something out). I have been to a lot of job talks at my university and did three job talks this year and there were questions during all of them. I agree that these are mostly short and sweet questions. The only time this can be negative is if someone perseverates on a question too much. In the event that that happens try to politely ask them to return to this issue during the longer question period.
Questions could be a sign of lack of clarity in your presentation or they could reflect audience engagement. I've received offers from schools where I was interrupted numerous times and I've bombed at schools where I wasn't interrupted once. From my experience it is almost impossible to decode what they mean. Just focus on doing the best talk you can. The rest is out of your control.