Have fun sometimes, just for the sake of it
The best way to create a safe learning atmosphere is to have fun on a regular basis. This does not mean that the classroom should be a comedy club or a circus. It does mean though that the teacher can keep positive energy running through the group by occasionally doing activities that are simply fun or at least good-natured, activities whose only aim is to create a good atmosphere in the room. Improv activities especially promote this idea of having fun while failing.
Two categories of entertainment
Entertainment takes many forms, and many of those forms involve professional people who take themselves very seriously. People like dancers, circus performers, singers, and usually actors all go through various forms of intensive training so that they can impress us with their skill. The result though is elitest. We watch these entertainers because they amaze us, we watch them and think wow, I could never do that.
There is another kind of entertainer however who come closer to what I think good teaching should be. These entertainers are comedians, talk-show hosts, and other entertainers whose skills are not technical, they are rather inter-personal. They inject humour and good-nature into a situation, they make sure that everybody is having a good time.
Be professional, but don't take yourself too seriously
I once saw a performer who is a great example of this. He started his act by singing to various famous rock songs whilst stripping through different layers of costumes that he had on. In the end he was just wearing a pink leotard which made him look completely ridiculous, as he was also a bit overweight and totally bald. The effect was to reassure the volunteers from the audience who would be later getting on stage with him. They clearly had no chance of looking as ridiculous as him. By being in charge, but not taking himself seriously he made the space safe for others. Was he professional? Absolutely he was. Should teachers do this? No, but there is a lesson to learn here.
A difficult rope to walk
I am saying here that teachers should not take themselves seriously, but that is obviously only true up to a point. A teacher is not a clown and it is important that their students can trust in them and their expertise as they lead those students through a course. A teacher also represents the institution where they work. However, a teacher who is too professional in this sense will not create the right kind of atmosphere for optimum learning. It is a paradox and an interesting balance for a good teacher to try and achieve.