When you choose to shoot portrait usually you will go for 85mm lens and more.It make sens. 85 mm lens is called portrait lens. And you get nice suppressed distortion and soft bokeh. But if you shoot every portrait with 85mm, all the portraits will have same result. Kind of boring. But don't get me wrong, 85mm is a killer lens. Must have for every portrait photographer. 50 mm lenses are also good for portraits, bit wider,good for every occasion, also must have lens. Now, 35mm i love this lens specially for out door portraits, amazing for street portraiture, when you need to get close to get your frame, better contact with your subject. Someone once said, "if you didn't get good picture, it mean you wasn't close enough".
But what if you shoot with 20mm or wider? First, you need to be really close to your subject, you want or not, you need to somehow connect to your subject. And that is good thing in photography when you need to connect.
On this portrait below i use 20mm lens f/2.8. I didn't want this portrait to be portrait by the book. If i shoot this portrait with 85mm and plus, i wouldn't get the story behind. By using 20mm i manage to capture my subject and also show the environment that goes with my subject. With 20mm lens i was able to show who the girl is on this picture. But be careful when you get to close with this 20mm or wider, you will get distortion. you will lose proportion.
So, my point in writing this post is, go out of the box and explore all the lenses for portraits.