Short answer: "Long lines" is a super simplification of Napoleonic tactics in larger engagements and company level engagements typically involved skirmishing by light infantry.

Long answer:

The line of battle was often very fluid, and a good commander will exploit and react to changes in the battle line.Skim through the following summary of Napoleon's tactics:http://www.napolun.com/mirror/napoleonistyka.atspace.com/infantry_tactics_4.htmYou'll notice a huge range of formations and tactics which do not characterize the period's warfare as simply "line up and shoot". I could go on and on about Napoleonic columns and how the "Grand Battery" strategem favored by the French created and encouraged fluid battle lines, but let's go back to your original question: battles on the company level.

Napoleonic era warfare was characterized by three distinct types of infantry roles:

Line infantry: Expected to define the frontal engagament line and prevent enemy breakthroughs.

Light infantry: Expected to define the flank and envelopment zones and prevent enemy flanking maneuvers.

Grenadiers: Elite and experienced troops expected to define anchors in the engagement line, to create and exploit breakthroughs in the enemy formation.

These were then expected to support and be supported by artillery and cavalry.

The French army was organized into companies, then sections, and each section was expected to react as a unit, with section officers using their own initiative to command their men. At the company level, officers would coordinate the positions of different sections to form a coherent front, with as much cohesion as the chaos of battle allowed. Each company was made up of homogenous troop types - so you had line companies, grenadier companies, and skirmisher companies.

Line companies and grenadier companies rarely moved independent of the army, but light infantry companies were typically expected to move ahead of the army for scouting and harrassment.

For the French, these were the Voltigeur, but all major nations had their own corps. Light infantry were expected to skirmish. They were expected to be mobile, quick, and often fought out of formation. They were armed with long range weapons (such as rifles) and were often deployed far ahead of major engagement zones.

The goal of light infantry was never to hold the line, but to provide accurate fire across long distances to harrass and pin down the enemy. Often, light infantry companies would meet each other in a skirmish battle, and typically, one of several things happened:

One side was often in enfilade, as would be the case in, say, stumbing upon a defended village, and the other would often be in defilade, such as a light company falling back to defend said village. In this case, the force in enfilade would simply retreat, and perhaps come back with line infantry and a few artillery pieces.

If both met in an open field, then the company farther away from their own forces would often retreat on contact.

If both sides had their main forces nearby, then they would engage while their respective armies began to deploy.

Point is, skirmishers tended to not get bogged down in fights, unless it was part of a larger, regiment level engagement, or if they were sure, and damned sure, they can win the day against the opposition.

There are, however, several examples of light infantry skirmishes which were part of larger engagements, where both sides used light infantry to capture and hold key objectives away from the main engagement zone. Two best example are:

Utitsa at Borodino, where Russian Jaegers contested the village against Polish light infantry (the Poles had artillery support, so it was not exactly an even fight, but anyway)

and more well known, Plancenoit at Waterloo, where Prussian light infantry attempted (and succeeded) to outflank Napoleon's forces.

For fun fun reading time, I recommend 3 books:

Napoleon's Line Infantry (Men at Arms Series, 141) by Philip Haythornthwaite

https://www.amazon.com/Napoleons-Line-Infantry-Men-Arms/dp/085045512X

and

The Campaigns of Napoleon Hardcover – March 1, 1973 by David G. Chandler

https://www.amazon.com/Campaigns-Napoleon-David-G-Chandler/dp/0025236601

Last, for posterity, I recommend the grandaddy of all things Napoleonic warfare:

On War by Carl von Clausewitz