I’m a big fan of experimenting with a variety of materials in my sketchbooks and art journaling. For me, learning how a materials is expressive is part of the journey of making art.
Every tool has it’s own mark and how the artist uses that tool is how it can become an expressive tool.
For this exercise I grabbed 4 random tools off my messy work surface: pencil, fat paint marker (15mm), brush tip marker, and a bold 1.0mm Uniball 207 black gel pen. I then headed over to EarthsWorld on instagram and decided to draw this woman 4 times.

Here are the spreads in my sketchbook. Each one is unique and as artists we can use the marks our tools make to capture different feelings and looks in our art. 
In my opinion the pencil feels unfinished as a sketch. It’s loose and lacks depth.
The black thick paint marker is bold and graphic. It lacks the detail of the pencil and finer materials but it has punch that would allow it to stand out on a brightly colored background, where the pencil would wash away. The thick black paint marker is used on it’s corners and edges to get a variety of lines, not just a simple flat black line.

The brush tip marker is purple, and it layers. So there are darker areas of purple and lighter areas. It has the punchiness of the black but more depth to the darker areas. It’s less flattened. If the marker were gray it would stand out on a lot of colored backgrounds. The brush tip gives the image a softness that the thick black paint marker lacks. There are more details here too.
Finally (for this experiment) I used that bold Uniball 207 gel pen. Line many pens the line is a single width (though with experience I can tell you that if I get the right angle on the pen, I can get a thinner scratchy line that is broken up) The amount of detail possible with a pen like this is pretty unmatched, though to be fair I was not looking for perfect detail here, merely to capture SOME of the potential of a pen line. You can layer and cross hatch and get some remarkable details with a gel pen.
I think that using a combo of materials really unlocks mood and feeling when it relates to the look of a pen or marker or pencil. It’s worth noting here that the paper used can really change how a material looks and we respond to it. For instance paint markers on rough paper look a lot different than they do on this pretty smooth paper. Pencil looks different on rougher or colored papers. Some pens won’t make much of a marker on rough paper.
Materials need to match one another but also the mood you are attempting to explore.