Journals traditionally have been similar to diaries in the historical sense. Only recently (past 100 years) have journals taken a new form in the world of fictional literature.
While just about everyone worldwide has read or been read at least some portion of a journal similar to The Diary of Anne Frank, about a Jewish girl who hid with her family from Nazis in Germany, it is still a "true life" story, but it inspires others to write their own "journals" about fictional or real people.
Exercises abound in writing class (especially for children) where the students are asked to keep a journal for a week or so as if they were a famous person in history or someone they admire who is famous. Someone might keep a journal concerning England's Elizabeth the First while someone else might choose Joe DiMaggio. While this is all well and good, and gets those creative juices flowing, it's still not fictional throughout. Elizabeth the First did exist and Joe DiMaggio really did hit that home run.
Creating a journal for fictional character might seem a bit easier, no pesky historical details to get absolutely right, but in fact, it should actually be harder. A well known personality is a well-known personality, everyone knows who Marilyn Monroe is and what she looked and acted like. Its well-documented on film, books and magazines. Someone you've made up in your head is something else entirely different. Through this journal, you have to make the readers care; they don't automatically care because they are familiar with the journal writer.
For example, using one of my (Wolfie) characters, I will create a journal entry or two. We all know who Albus Dumbledore is and we know who Sirius Black is, but you know absolutely nothing about Adara Pycron (other than what I've put in the character sketch some weeks back, so no peeking!!). (Oh and no griping about the obvious LotR overtones...it's not my fault things subtley started shifting that way after Fellowship of the Rings came out in theaters!!)
Day 516 of the Second Lunar Season of Minair Regnal Calendar
The trees spoke of a stranger walking the paths of the woods. After doing some scouting of my own, I determined that a lesson should be taught. I took my initiates to the Guardians of the Forest with me and we encircled our unsuspecting intruder. In fact, he may have known we were there, but he still could do little to counter us. He's been here often enough that he knows how strong Elves are in this land. Needless to say, it was indeed quite the surprise when I snuck behind him and disarmed him of his own wand. The joke was well played and two lessons learned. Unfortunately, Dumbledore's visit was not a social one, more's the pity, for I enjoy our sparring greatly. He has asked me to return to Man's World. My father has agreed, with the backing of the Lord and the Council. This new dark wizard must be countered before he reveals the existance of us all to non-magical men.
Day 522 of the Second Lunare Season of Minair Regnal Calendar
The Forbidden Forest is looking poor. I have taken Dumbledore to accounts for it's mistreatment and he was puzzled. Can he not see the death surrounding the trees? I spoke to the centaurs, vague and irritating creatures though they may be, and they agree that more stringent measures must be taken. I have resolved to teach the Herbology teacher and Rubeus Hagrid, the school's rather amusing half-giant grounds keeper, a few magical tricks to make the grounds more healthy for the trees that sink their roots in.
I must also confess a sin. As we walked up the path to Hogwarts (dear place, it never changes!) four boys were by the old lake. They greeted Albus most fondly. One was most handsome, even so young. I daresay that he will even more so when he is older. I cannot help but wonder this, even as I shudder to think why. Elves and Men are not to be together; the tales of Mistory Highwind and Gavin MacAtee are more than a strong warning against such unions. I have no wish to live my life to the end of Man's span, never joining my family in the White Mists Beyond. Yet still...Albus said the boy's name was Sirius. I found it appropriate that both he and I were named after stars in the same constellation. It reeks of a terrifying fate I dare not think more upon.
Day 541 of the Second Lunare Season of Minair Regnal Calendar
Albus was correct. This dark wizard is indeed more powerful and dangerous than Grindelwald. Why do men, magic or no, feel the urge to destroy one another? This I do not understand. Rumblings from home (sent by my hawk Ra) tell of the trolls wanting to come to the surface. If they do, they will destroy everything, for they have no respect for life. My loyalties are torn, but I have given my word. With Albus, Hogwarts and Man I will stay - for now.
As you can see (briefly) the plot is starting to shape. (I'm not writing the story in journal form however, so too bad) Perhaps more fleshing out between, etc. will create a more poignant setting. Also, you can begin chapters with journal entries to give the reader an idea of events.
Journals make great contrasts within the story as well. Start the chapter with your character's point of view through the journal, then redo some of the events in the chapter as the story progresses through other characters point of view, both allied and opposed to give a more real, 3-D, fleshed out story. This allows the reader to have more or less sympathy with whomever you want and thus progress the suspense and plot much easier.
Besides, it's actually kinda fun!!!