Alma was just driven out of Ammonihah for preaching the gospel. The scriptures say the people "reviled him and spit upon him." I imagine Alma was kind of angry (how could someone spitting on you NOT make you angry) and a lot discouraged.
vs 15-- the same angel who visited Alma when he was running amok, driving people away from the church visits him again and says, "Blessed art thou, Alma; therefore, lift up thy head and rejoice, for thou hast great cause to rejoice; for thou has been faithful in keeping the commandments of god from the time which thou receivedst thy first message from him..." My thoughts-- when life is hard and we become discouraged, we can take comfort in the fact that we are doing what the Lord has asked of us. I think that applies to the basic commandments-- the big 10, attending church, reading our scriptures, praying, etc-- as well as individual "assignments"-- magnifying a really crappy calling, raising a sick kid, accepting the death of a loved one.
Vs 18-- in between vss 15 and 18, the angel tells Alma to go back to Ammonihah and preach again. (Every time I read this part my mouth hits the floor.) "Now it came to pass that after Alma had received his message from the angel of the Lord he returned speedily to the land of Ammonihah." My thoughts-- What a guy! Alma doesn't just go back, he goes back "speedily." That's why he's a prophet.
vs 31-- in the previous vss, Alma picks up Amulek to be his missionary companion. "And they (speaking of Alma and Amulek) had power given unto them, insomuch that they could not be confined in dungeons; neither was it possible that any man could slay them; nevertheless they did not exercise their power until they were bound in bands and cast into prison. Now, this was done that the Lord might show forth his power in them." My thoughts-- They were given power so that the people could see they were of God. Before reading that last sentence in that verse, my thoughts were drawn to 1 Nephi 3:7-- this is after the Lord commanded Lehi to send his sons back to retrieve the brass plates-- "And it came to pass that I, Nephi, said unto my father; I will go and do the things which the Lord hath commanded, for I know that the Lord giveth no commandments unto the children of men, save he shall prepare a way for them that they may accomplish the thing which he commandeth them." HF gives us "power" and prepares the way for us to accomplish whatever he asks us to do.