We often use the present simple and present continuous in stories and jokes in informal spoken English to create the impression that events are happening now. This can make them more direct and exciting and hold people’s attention (M. Hewings, "Advanced Grammar in Use" p. 4, 2013)
I have read and even posted an almost identical question related to this problem, but in this question, I want to ask as to how to implement this idea. First and foremost, I think I should give an example from the very same book I quoted above.
I was sitting in the park, reading a newspaper, when all of a sudden this dog jumps at me.
If this were the first time I saw that sentence before I read Hewing's book, I'd say, in my opinion as a learner, this sentence would be incorrect since they use different tenses at the same moment. I implied "at the same moment" because the story teller used "all of a sudden" that made me think the first and the second event weren't separated by a long interval.
Corresponding to the title, suppose I want even more to hold people’s attention, would this sentence sound ungrammatical if I write this way?
I am sitting in the park, reading a newspaper, when all of a sudden this dog jumps at me.
Do the listeners (of the story teller) would understand that the teller is talking about something that did happen in the past? Or would they rather be comfortable if the tense is using the past tense instead of the present tense?