- whether they mention the salary offer on a job description
- whether you apply for a job in a company, or with a recruitment agency that can send you to work to different places
- whether they ask about your salary expectations in the first (screening) interview, or at the end of the second (or final) interview with you
- whether you apply for a simple manual labor, a managerial role, or perhaps a highly technical job
- whether many people apply for the same position, or you are the only candidate
The more they want (or need) you, the more you can ask for
If you apply for a manual labor (assembly line, cashier, stocker), or for one of the popular jobs (administrative assistant, secretary, pharmacist, receptionist, etc), they can easily screen you out, for any reason (including high salary expectations), simply becasue they will always have another applicants to hire.
Oppositely, if you offer a unique skill or knowledge, or if you apply for a job nobody else want to do, simply if you are their only option, you can definitely ask fort more money. It is much easier to negotiate a better salary in this scenario.
Another thing to consider is whether you talk to a decision maker in an interview, or just an ordinary employee. An HR generalist, or trainee recruiter can hardly offer you more money–they are not authorized to do that. Oppositely, if you already passed two interviews, and find yourself talking to a high executive from the company (CEO, CTO, CFO, etc), you can have higher expectations, becasue if they invited you for the final interview, they seriously consider hiring you (they want you in their team by then).
As you can see, this questions requires some analysis from your side. Analyze the situation and decide how much money you can ask from them.
A safe way of answering the questions
A safe way is to emphasize that salary is not a deciding factor for you, that you really want the job, and will accept whatever they offer to new hires (you can still refuse their offer once they share it with you).
You can elaborate on such an answer, citing how much do workers on the position earn in average (you can find the information on many websites, for example indeed.com), and say that you will be happy getting the average salary.
You can also mention some benefits (company car, laptop, mobile phone), but be sure to check if they are standard for the position you try to get.
Have something to backup your claims with
Most often than not, you will find a salary range on their job offer. If they write from $30,000 to $35,000 annually, you should not ask for more than $35,000. In an exceptional case (when you apply for a specialist job), you can add ten or twenty percent to their offer ($35,000 + 10%).
One way or another, the interviewers should not get the feeling that salary is the deciding factor for you. Try to keep your claims realistic, consider the situation and decide how much you can realistically get. That is the best way of answering this particular question.