Definitely be friendly, open body language, smile. Ask a non-interview question or make a comment to start with - about their journey in, the weather, compliment their scarf, whatever. You’ll always get a better idea of people’s abilities if they can shake off the nerves. If you know from their CV you have something in common then mention it early on while settling in - same uni, same former employer, anything that helps relax them.
Have a list of central questions that you ask all of them - the really crucial stuff, like experience with specific programs, skills in key areas etc - that you can mark and rank. Make a list of more peripheral questions that will help you gauge their personality and competency. For each candidate, note down any queries you have about gaps in their experience or skills after reading their CVs.
Come up with a few “what would you do if...” scenarios to throw at them, relevant to the role.
If you are able to set up an informal test, do so - some people are excellent bullshitters but then useless, others perform poorly in interviews but will impress with their skills.
If you’ve selected them for interview it’s likely they have the skills and experience to fundamentally do the job. You need to work out, outside of this, what attributes are really important in a new hire - maybe you have a team with some tricky personalities and you need someone easy going, maybe you need someone who can work well with a specific team member so they need to complement their skills, maybe it’s someone who is used to working independently with little oversight and therefore has really good time management and self motivation skills... whatever it is, figure out what you need and the questions will become clear.
And don’t ask what their weakness is - it’s a stupid question that’s never answered honestly, gets a rehearsed response and is just pointless.