What was he doing all night?

[First Samuel] Chapter 15, among other things, illustrates what Samuel meant when he declared that he, as a prophet, considers it as sin (חטא [ḥt']) against the Lord not to intercede for those entrusted to him (1 Sam 12:23). In characteristic fashion, the prophet is not only informed about Yhwh’s intention but is also commissioned to deliver the will of God (cf. Amos 3:7). In this case, Samuel is sent to inform Saul that God has rejected him as king because of his disobedience. In response to Yhwh’s strict words, Samuel spends the entire night in prayer with the Lord (1 Sam 15:11). Chapter 15 raises issues of great theological delicacy, such as the power and discernment of genuine repentance, election, and covenant obedience, divine mutability, and the limits of prophetic intercession.—Standing in the Breach, page 177

<idle musing>
What was Samuel doing all night? Another model of what a true prophet looks like. He's given a message to deliver, but before he delivers it, he spends the whole night interceding, asking God to be merciful—at least that's how I read it, based on Samuel's comment in 1 Sam 12:23. That doesn't seem to be the way some of these so-called prophets work today.

Even if you can get them to say anything other than "God will bless you with abundant material blessings," all they will do is stand on a hill and pronounce curses.

I know of exceptions, real prophets who almost sweat blood interceding, but they are the exception, not the rule.
</idle musing>