How can Douglass's tone be both cool/reserved and angry/emotional? Well, overly emotional narrators don't always come across as reliable. Since Douglass wants to convince us that he's just telling the truth about what happened to him, he mostly tries to keep his anger about slavery hidden. At the same time, Douglass is really angry about slavery and he wants us to be angry too. So even though he keeps his anger in check most of the time, every once in a while he'll let us know how he really feels.
Here is an example where Douglass really lets his emotions run free. It's from the famous speech he gives when he looks out at the Chesapeake Bay and wishes he could be as free as the white sails of the ships:
"You are loosed from your moorings, and are free; I am fast in my chains, and am a slave! You move merrily before the gentle gale, and I sadly before the bloody whip! You are freedom's swift-winged angels, that fly round the world; I am confined in bands of iron! O that I were free! O, that I were on one of your gallant decks, and under your protecting wing! Alas! betwixt me and you, the turbid waters roll. Go on, go on. O that I could also go! Could I but swim! If I could fly! O, why was I born a man, of whom to make a brute! The glad ship is gone; she hides in the dim distance. I am left in the hottest hell of unending slavery. O God, save me! God, deliver me! Let me be free! Is there any God? Why am I a slave? I will run away. I will not stand it. Get caught, or get clear, I'll try it. I had as well die with ague as the fever. I have only one life to lose. I had as well be killed running as die standing. Only think of it; one hundred miles straight north, and I am free! Try it? Yes! God helping me, I will. It cannot be that I shall live and die a slave. I will take to the water. This very bay shall yet bear me into freedom. The steamboats steered in a north-east course from North Point. I will do the same; and when I get to the head of the bay, I will turn my canoe adrift, and walk straight through Delaware into Pennsylvania. When I get there, I shall not be required to have a pass; I can travel without being disturbed. Let but the first opportunity offer, and, come what will, I am off. Meanwhile, I will try to bear up under the yoke. I am not the only slave in the world. Why should I fret? I can bear as much as any of them. Besides, I am but a boy, and all boys are bound to some one. It may be that my misery in slavery will only increase my happiness when I get free. There is a better day coming." (10.8)