There is no pain, except for the initial prick of the needle. That can be avoided too, if an intravenous catheter is already in place due to an illness. Sometimes the animal may sigh, move, or void. These are just reflexes. The animal doesn't feel pain - or anything whatsoever.
Death follows unconsciousness within two minutes.
Euthanasia can be very emotional for veterinarians as well. Sometimes the veterinarian has known the pet for a long time or has worked hard in trying to prolong the life of the pet. Grieving will be a natural and necessary part of accepting the loss of a pet. Many support groups have been organized within communities, the Internet community, veterinary schools, and local mental health organizations to help people go through what can be a very painful process. Your veterinarian may know of a local support group or individual who provides counseling.