I have been helping my company interview, which makes for a great opportunity to elaborate on my experiences and perspectives: what do I look for when I interview a candidate?
Green flags
These are a collection of positive qualities and behaviors that I look for in a candidate.
Online presence
Typically the first impression for a candidate is a resume. A clean and straightforward resume is ideal: don't over-embellish. Next, I'll look for an online presence, such as a Github account and a personal website/blog. Bonus points for linking the accounts and websites in the resume.
I'll browse the candidate's repos and blog posts, looking to get a feel for the candidate's experiences. When possible, I'll ask the candidate during the interview about personal projects or blog posts - a good way to build rapport and demonstrate interest.
I don't expect anything excessive: a couple personal projects that are online or documented are sufficient. A website and a blog with a couple blog posts discussing their experiences is icing on the cake.
Technical competence
During the interview, I am looking for competence, not necessarily expertise or excellence. That's not to say the bar isn't high, but that in a 1 hour interview there's only so much a candidate can do!
To that end, I prefer to ask broad questions, i.e. toss softballs, hoping the candidate hits a home run. For example, if a candidate has used Django and Laravel, I could ask to compare the frameworks. A good answer would prove that the candidate has actually used the frameworks; a great answer will explain in-depth pain points or unique qualities of each. Open-ended questions are a chance for a candidate to demonstrate their experience, in a format more comfortable than trivia questions.
Red flags
Green flags wouldn't be complete without red flags: qualities that sink a candidate.
Poor communication
This is one of the first pieces of interview advice I learned, but it bears repeating because I've witnessed the mistake: communicate during the interview! If you're doing a coding exercise, explain your thought process, talk about the code that you're writing, ask questions - the interview is as much about understanding how you work as what you can produce. Even if you fail to complete the exercise, solid communication may score you ahead of a candidate who finished in silence.
Poor attitude
Similar to communication is attitude: be a good sport. Get into a good headspace before an interview by taking a walk or meditating. Advocate for yourself by taking breaks between sessions. Everyone has bad days - it may be better to reschedule than be grumpy.
This isn't to suggest putting on faux-excitement over a job you're not excited about. You can show interest by asking good questions about the role and the company, and that's sufficient.
Conclusion
These are just a few of my perspectives on interviewing, colored by experiences on both sides of the table.
One key piece of advice: interviewing is a skill, practice makes perfect.