True. Many people never speak their heart out. They will have something in their mind but they talk something outside. They may be very friendly to you for some reason. Why Karna is a friend to Duryodhana? Duryodhana is aware that Arjuna is a great warrior and none of their brothers can compete with him. So when he has seen Karna he thought he will be useful to him. So he became a friend to him by giving Anga Rajya. Karna accepted the friendship of Duryodhana as he has given him a Kingdom and made him the king. That means that there is some vested interest in their friendship.
A true friend will never expect anything from his friend and at the same time, a real friend will never hesitate to give anything he can give to his friend when there is a need. But such friends are very less. we should be able to identify the real friends and we should always try to be a real friend.