I'm going to assume that you already know your characters' name, and a rough idea of what they look like. But here are some ideas to get you thinking more deeply about who they are – and of course, ignore anything that you don't like or isn't relevant.
1. Do they have any tattoos or piercings?
2. What kind of clothes do they wear?
3. Any scars? How did they get them?
4. What family do they have?
5. What is their relationship with their family?
6. Place of birth?
7. Where do they live?
8. Where have they lived?
9. What is their job?
10. Do they have any specialist training? Qualifications?
11. Do they have a natural talent for something?
12. Do they like to travel?
13. Where is their favourite place?
14. Who is their best friend – or friends?
15. Who was their best friend/s as a child?
16. What happened to those friends? Are they still in contact?
17. Does this character like him/herself?
18. Are they in good health?
19. Any minor health issues?
20. Favourite food?
21. Food they don't like?
22. Favourite book?
23. Favourite soft drink?
24. Alcoholic drink of choice?
25. Favourite movie/film/play?
26. Any hobbies or pastimes?
27. Do they play any sports? Are they any good?
28. What might this person do in their spare time?
29. What would be a great gift for this person?
30. Do they have any pets?
31. Favourite childhood pet?
32. Can they drive? Do they have a favourite vehicle?
34. Most embarrassing thing that ever happened to him?
35. Are they more optimistic or pessimistic?
36. Are they introverted or extroverted?
37. How do they display affection?
38. How do they react to praise?
39. How do they react to criticism?
40. What is their greatest fear?
41. When was the last time they cried?
42. Would they prefer to be indoors or outdoors?
43. What is their best childhood memory?
44. What is their worst childhood memory?
45. Are they in a relationship?
46. How do they behave in a relationship?
47. Has your character ever been in love?
48. Have they ever had their heart broken?
49. How do they respond to a threat?
50. What is their choice of weapon?
51. What is their favourite animal?
52. Which animal to they dislike the most?
53. What is the most beautiful thing they have ever seen?
54. What is on their bedside table?
55. What is in their bag/purse/wallet/pocket?
56. What is their most treasured possession?
57. Are they minimalist or a clutter hoarder?
58. What do they do first thing on a weekday morning?
59. Are they comfortable with technology?
60. What is their most powerful memory?
61. What might they have for breakfast?
62. Can they – do they like to – cook?
63. What would you notice first about this character?
64. Favourite childhood toy.
65. Have they killed anyone? Who was the first?
66. What would be a good way for them to spend a day?
67. What was their first experience of death? Who, or what, was it?
68. Do they have a hero or role model?
69. Do they like music? What kind?
70. Do they have any birth marks, moles or freckles?
71. Favourite cafe/restaurant/pub/public place?
72. Do they take any regular medication, or have any allergies or intolerance's?
73. Is there something this person can't, or doesn't like, to do?
74. Does this person have a dream or goal to achieve?