Rashi (from Bamidbar Rabah 23:9): It evaded them and went under Eliyahu's garment, for it did not want to be offered to idolatry. He told it 'go - through both of you, Hashem will be sanctified!'

Radak #1: He let them choose one.

Radak #2: Achav gave to them.

Radak citing Tanchuma Mas'ei 8: They could not budge it until Eliyahu told it to go. The bull said to him, I and my colleague left the womb together. We grew up on one feeding trough. He will be offered to Hashem, and Hashem will be sanctified through him, and I came to the Ba'al's portion, to anger my Creator?! He told it 'go - just like Hashem will be sanctified through the bull with me, also through you!' It said 'do you counsel me so? I will not budge until you deliver me into their hands!' ¹

Malbim: Since the bulls were not the same (refer to 18:23:1:1), and he told them to choose [the better one], it is as if he gave to them the better one.

Malbim: Ba'al was Avodah of the sun. It is strongest from morning until afternoon ¹ .

Malbim: The Siman of acceptance of the Korban was a voice of Nevu'ah, or fire - "Asher Ya'aneh va'Esh" (verse 24). Neither came.

Rashi citing Targum Yonasan: They acted crazy; they danced like [idolaters] normally do on their Mizbechos. Va'Yfaschu is like va'Yifse'u.

Radak: They made themselves as if they were lame and in pain, going on the Mizbe'ach.

Malbim: They destroyed [the Mizbe'ach that Eliyahu built immediately, for they thought that it ruined their Avodah]. This is why it says "va'Yrapei Es Mizbach Hashem he'Harus" (verse 30).

Yalkut Shimoni (214): Chi'el (who buried all his sons while rebuilding Yericho - 16:34) made the Mizbe'ach hollow. He hid inside, and was ready to make a fire when the Nevi'im called. Hashem sent a snake to bite him; he died.

Radak: This is like Asher Asu, but it refers to the Klal; Yisrael made it.

Malbim: Eliyahu made [the Mizbe'ach on which he would offer].