Italics are probably the most misunderstood style in type design, but they are also the style with the greatest potential excitement and fun due to the large number of variables for you the designer to play with.
Italics are different from bolds in that they are not meant to appear to have a different weight than the regular. Instead, they are meant to offer a different texture than the regular. Greater intensity in this difference will mean that the italic is especially useful for creating a sense of contrast with the regular. This stronger effect is useful for highlighting single words or short passages of text. In contrast, a less-different texture is often useful in situations where you are setting multiple lines, whole paragraphs, or even even pages in an italic.
The variable most commonly associated with italics is slant. Indeed, when a web browser is asked for an italic in a CSS rule and there is no Italic, it will simply slant the regular to create a synthetic or faux italic. It is probably not surprising that when people first begin designing type they also consider this approach. The origins of this idea go back to the mid-20th Century and modernism as it was applied to design. This is why the first italics seen in typefaces such as Helvetica were also slanted versions of the regular.
How much slant?
Some italics have no slant. No, really! These italics are called upright italics. However it is likely that if your design has only one italic in it that you will choose for that italic to have some degree of slant. In general, italics tend to slant between 4-14 degrees. Most contemporary fonts slant between 6-9 degrees.
While slant can be important to the design of an italic, is easy to notice, and can even be done with some limited success using an automatic filter, it is not the only variable that you can use to help separate your italic from your regular. You may want to consider using one or more of the following variables in addition to slant.
The term "italic" does not in fact refer to the slant seen in in many italics designs but instead refers instead to a style of writing which became popular in 14th century Italy. This style of writing was a faster and a connected form of writing which uses a different construction for its letters than is seen in regular. This different construction or pattern of strokes is what type designers are referring to when they say they have designed a "real" or "true" italic. This construction has many sub characteristics that you may choose to include in an italic design.
The most obvious of these characteristics is the triangular countershape created by letters with joins. These letters include a, b, d, g, h, m, n, p, q, and u. This variable is powerful, partly because countershape is a powerful variable, but also because of the great number of letters with the feature. That fact, combined with the high frequency of their usage in most languages, is also a very large (and probably the even greater) factor.
When designing your italic, you can very effectively tune the effect your italic gives by making relatively small adjustments to the height of the joins. Subtle changes can give surprisingly large results. Still, not all italic fonts take advantage of this variable.
In and out strokes
Many italic fonts make use of asymmetical serifs, in the form of in- or out-strokes, or both. When only one is used, it is more common to use the outstroke and to have an upright style applied where the instroke might have been. The intensity of the effect that instroke and outstroke has can be controlled by the weight of the strokes and by adjusting how long they are. Like triangular counters, a great part of their utility and power comes from the fact that so many letters use them.
Italics are normally somewhat less wide or more condensed than the regular style. Because condensation is a feature seen across all of the letters in the italic, it is a very powerful variable. This variable can be employed in both a gross and subtle manner. If you choose to use this variable, it is necessary to adjust the weight of the strokes to make the italic appear to be the same or nearly the same weight as the regular design. The more condensed your italic is, the more you will need to make this adjustment.
Most italics use all of the variables listed above in various proportions. You may find that it is useful to look at a range of italic designs and analyse which variables are being used and in what strength. When you do this, you will notice that none of the variables are used at full strength. Instead, one of the variables tends to lead, with some limited use of the others. The stronger the use of the variables the more contrast your italic will have from the regular.
In is also notable that in the last ten years we have seen an increasing number of type designers choose to offer not just one italic in their type families, but two or even more. It is also notable that dictionaries sometimes make use of more than one style of italic.
When they were first made for printing, italics were not thought of as part of the same type design or type family. This idea is one which became standard over the 19th and 20th Centuries. Even the idea of mixing italics with regular was not part of the original idea behind this script. The first italic fonts were used to set entire books, instead of the upright roman style. It is probably safe to assume that the role of the italic will continue to evolve.