This is quite a relevant question. Think about how much debate right now centers on how movies and TV shows and the like impact us, the audience. That's what Horace and Aristotle were concerned about too.

Concerning Aristotle, one place you might want to start is the idea of highbrow versus lowbrow culture. In our culture, we tend to view some things as having more value than others. For example, a painting in a museum might be deemed more important than a tweet. Yet why should a painting in a museum hold more value than a post on social media for an audience? Shouldn't the audience decide?

Aristotle collapses conventional notions of the "cultivated audience" and the "unrefined" viewer when he discusses the epic versus the tragedy. Aristotle writes how people think of the epic as "higher" since it doesn’t need "gesture" like the tragic. However, Aristotle notes that "gesticulation may be equally overdone in epic," and "all action is not to be condemned, any more than all dancing, but only that of bad performers.”

Here, we see Aristotle treating the audience as thoughtful and mindful regardless of what they're watching. If the impact isn't what the artist intended, instead of blaming the audience, maybe blame the artists themselves.

Like Aristotle, Horace feels it’s the obligation of the artist to impact the audience. "It is not enough that poems be beautiful," writes Horace, "let them be tender and affecting and bear away the soul of the auditor whithersoever they please."

Later on, Horace tells the hypothetical writer, "If you would have me weep, you must first express the passion of grief yourself."

Again, with Horace, as with Aristotle, the spotlight is on the artist. It’s up to them to move, lift, and transcend the audience. If they fail, it's not because the audience is necessarily dumb or dull. It’s more likely because the work itself is faulty in some way.

Aristotle views the effects of literature on the audience in a number of ways, including the following:

• He suggests that human beings enjoy learning and acquiring knowledge. Poetry appeals to this innate human instinct to learn. Plato assumed that most people are uninterested in learning. In contrast, Aristotle assumes that the instinct to learn is very deeply embedded in human nature.  As he puts it in Part IV of The Poetics (in the Samuel Butcher translation),

. . . the instinct of imitation is implanted in man from childhood, one difference between him and other animals being that he is the most imitative of living creatures, and through imitation learns his earliest lessons . . . . to learn gives the liveliest pleasure, not only to philosophers but to men in general . . . .

• Aristotle assumes that audiences expect works to be complex unities in which all the parts fit together to produce a complicated but coherent whole.  Works that satisfy this desire for complex unity give pleasure to audiences. Thus Aristotle is explicitly concerned with the aesthetic pleasure a work of art can give.
• As he puts it in Part VI,

Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament, the several kinds being found in separate parts of the play; in the form of action, not of narrative; through pity and fear effecting the proper purgation of these emotions . . .

“Catharsis,” the word which is translated here as “purgation,” has been interpreted in significantly different ways over the years. Perhaps the most persuasive interpretation of Arisotle’s meaning can be found in the edition of the Poetics prepared by Leon Golden and O. B. Hardison, who argue that the word really means something like “clarification,” so that the effect of a great tragedy is not completely emotional but significantly intellectual.

Horace’s theory of poetry, as expressed in his Ars Poetica, takes for granted many ideas similar to those found in Aristotle’s Poetics. Horace assumes, for instance, that a work must be unified to be effective, although his view of unity is simpler than Aristotle’s.  For the most part, Horace argues that the work of art must appeal to an audience if it is to be considered successful. Its appeal is often rooted in following custom and tradition, since these are codifications of what has proven successful in the past.

At one point in the Ars Poetica, Horace famously writes (in the A. S. Kline translation), that

Poets wish to benefit or to please, or to speak

What is both enjoyable and helpful to living.

Notice that Horace here gives poets three options: to teach, to please, or to do both. By the time of the Renaissance, these options had been combined, so that it was common by that time to say that poetry should teach and please or teach by pleasing.

Aristotle's Poetics provides a complex psychological account of literature. It argues that humans naturally derive pleasure from imitation, and thus that poetic mimesis is enjoyable in and of itself. Secondly. he famously states that through fear and pity tragedy achieves the catharsis of the pathemata, in some way purifying the audiences emotional responses. This is part of a larger argument, also advanced to a degree in his Politics and Nicomachean Ethics that art can serve to educate the emotions of the populace. Horace does less in the way of complex psychological analysis, but he does suggest that art should delight and be useful or instruct (dulce et utile).