Although Kṛṣṇa is the original person, one without a second, He assumes different forms just to satisfy His devotees or to execute a specific purpose.
(Srimad Bhagavatam----5:18:7----purport).

In the Eleventh Chapter of Bhagavad-gītā also it is stated that Arjuna prayed to see Kṛṣṇa's form of four hands (tenaiva rūpeṇa catur-bhujena). After revealing this form, Kṛṣṇa, when petitioned by Arjuna, again assumed His original humanlike form (mānuṣaṁ rūpam). These different features of the Supreme Lord are certainly not those of an ordinary human being.
(Bhagavad-Gita----9:11----purport).

As soon as one chants the Hare Kṛṣṇa mantra, he sees the forms of Kṛṣṇa, Rāma and Their energies, and that is the perfect stage of trance. One should not artificially try to see the form of the Lord while chanting Hare Kṛṣṇa, but when the chanting is performed offenselessly the Lord will automatically reveal Himself to the view of the chanter.
(Srimad Bhagavatam----4:8:53----purport).

My dear brāhmaṇa, austerity in the form of meditation is My heart, Vedic knowledge in the form of hymns and mantras constitutes My body, and spiritual activities and ecstatic emotions are My actual form. The ritualistic ceremonies and sacrifices, when properly conducted, are the various limbs of My body, the unseen good fortune proceeding from pious or spiritual activities constitutes My mind, and the demigods who execute My orders in various departments are My life and soul.
(Srimad Bhagavatam----6:4:46----translation).

By advancement in the association of the devotee one is placed in devotional service, and this service dispels all one's misgivings about Kṛṣṇa, or God, and Kṛṣṇa's activities, form, pastimes, name and other features. After these misgivings have been perfectly cleared away, one becomes fixed in one's study.
(Bhagavad-Gita----8:28----purport).

By offenseless chanting and hearing of the holy name of the Lord, one becomes acquainted with the transcendental form of the Lord, and then with the attributes of the Lord, and then with the transcendental nature of His pastimes, etc. Here it is mentioned that one should constantly chant the holy name of the Lord after hearing it from authorities. This means that hearing from the authorities is the first essential.
(Srimad Bhagavatam----2:11----purport).

Actually, in our present state it is not possible to see God in His spiritual form because our material eyes and senses cannot conceive of a spiritual form. We cannot even see the spiritual form of the individual soul. When a man dies we cannot see how the spiritual form leaves the body. That is the defect of our material senses. In order to be seen by our material senses, the Supreme Personality of Godhead accepts a favorable form which is called arcā-vigraha.
(Srimad Bhagavatam----3:25:35----purport).

There are hundreds and thousands of Viṣṇus, but for a devotee no form of Kṛṣṇa is important but the original form, two-handed Śyāmasundara. In the Brahma-saṁhitā it is stated that those who are attached to the Śyāmasundara form of Kṛṣṇa in love and devotion can see Him always within the heart and cannot see anything else.
(Bhagavad-Gita----11:55----purport).