I've been thinking a lot about minor characters. Readers rarely get into their heads (unless they're talking to an MC, and even then things might be edited) and yet I've always felt like the books I've LOVED rather than liked in part have been because of minor characters I cared about as much as the major players. Some of these include Harry Potter, the Hunger Games, Pride and Prejudice (Jane Austen in general), and many others.
So I've asked myself, what makes their minor characters so lovable? This may not be THE answer, but it's my answer. And that is their personalities are so strong, they compete with those of the MC.
Let's look at Harry Potter, which has a huge cast of minor characters. SPOILERS.
Who cried when Fred died? (everyone's hand should be up now!) Why was his death so sad? Because whenever he graced the page, he made us laugh. His goofy behavior came through with one line of dialogue, and we saw how brave he was as he fought in the battle.
What about Dobby? That funny little elf had his own way of speaking, an earnestness and desire to help that he became endearing to the reader, and his death also brought many, many tears from the readers who over 6 books loved him.
Tonks? Her quick replies, confidence in herself, loyalty, and bravery jumped out on each page (well, except the 6th book when she was mopey, but at that point, we cared about why she was so sad!).
With minor characters, they need a ZING to their personalities. You don't have a whole book dedicated to their arc, you have a few conversations with the MC. Make the reader love them because of their vibrant and apparent personalities.