In real life, each person is unique.
In a well-written story, each character is unique, too. Each one has his own fingerprint. Each one has her own voice.
I don’t know about how it is for you, but I tend to make all my characters sound the same. If you fall into that trap easily, too, here’s a writing exercise to try:
1. Pretend all your characters in your story are flowers. Each one is a different flower.
2. Pick a different flower for each one of your characters to be.
3. Write a short bio of each character based on the flower you’ve picked for it to be. For example, if Grandma (in your story) is a rose, she can be very wealthy and regal. She can be famous. She can smell wonderful with some mysterious perfume she wears. Everyone smells it as soon as she enters a room. People adore and revere her. But to her granddaughter, she’s all thorns.
4. After each character has their own unique bio based on which flower he or she is, ask each one to answer this question in his or her own unique voice:
There’s a knock on the door…who might it be?