What Do You Do When Your Characters Are Shallow?

I recently discovered that most of my main characters were all very similar. Not knowing what to do about it, I asked some other writers I know online and they gave me some really great ideas that I would like to share with you.
So what causes characters to be shallow?
There can be a lot of reasons your characters are underdeveloped. For me the problem was that I made them too much like myself. Including little bits of yourself in your characters is fine, but making one exactly like you is probably not the best idea. If you are, say, an introvert, then you can relate to your characters by making one an introvert. But don't make all of them introverts for two reasons.
One: It will be difficult for them to interact with each other if they are all shy.
Two: They are all too similar.
Another issue that causes characters to be shallow is the fact that you may not know how to deepen them. And that brings me to my next point.
How to deepen your characters.
One of my friends gave me the really great idea of interviewing them. Asking them questions and having them answer them. Not questions like "Do you like chocolate or vanilla." Those can be helpful sometimes, but it is not getting to the heart of your character. Ask them questions like "If you could go back and redo something in your life, would you? And what would you redo?" Deep questions that let you know more about your characters. Or you could have someone else interview your characters. When I did that, the person asking my character questions asked some really deep questions that I would have never thought to ask her.
A few more ideas are having them write 'journal entries' or just have their thoughts fill up a page or two. You can also pretend to be in their situation. This might not be something you want to do in front of other people because if you are thinking out loud, it might look a little odd. But if you are co-authoring something with someone, or if you have a friend who is a writer, you might be able to do this with someone else. I've often gone outside to act out what is happening in my WIP either to get rid of writer's block or to develop my characters.
So, how does having deep characters impact your story?
It makes your characters so much more relatable and the reader can sympathise with them a lot better. The reader can laugh and cry with them and understand them. They have a reason to root for them and they can connect with them on a much deeper level.
I was reading a book (a series of books actually) called The Wingfeather Saga. The characters were so deep and almost seemed real that by the time I finished the book I was actually crying with and for them. Even the villain was developed and had a reason for his deeds. I won't spoil the book for those who haven't finished reading it yet, but it had such a good ending.
Well, that's it. I hope this has been informative and has given you some ideas to develop your characters.

What do you think? Have you used any of these tips in the past? What do you do to deepen your characters? If you like, comment.

P. S. Here are some character questionnaires that I found.