Guru is the one who shows us the way to God. Guru must be full of enthusiasm to teach to a worthy student. He should not postpone a learning session , he should not divert an issue out of ignorance. He should not feign ignorance on critical topics. He should not delay the answer to inquisitive question.
When both guru and God approach ,it is better to salute guru first , since if God is angry on us Guru can bring him back for us. But if guru is angry , even God does not help.
There are twelve gurus, Laxmi Narayana, Bramha Saraswati,Vayu Bharathi, Vedvyasa , Srimadanandathirtha, Moolaguru,devguru,adiguru,paramguru,atmaguru,Mata,Pita,swaguru.
others are vidyagurus who happen to teach us on certain occassions like school. All of these are important and respectful. A propitiation to all these before start of any new learning definitely gives success.