I can't say much about the Allied intervention, but I can say that the battles on the eastern front were fairly fluid-and very bloody!

You had battles like Tannenberg, Lemberg, and Lodz in 1914; the Gorlice-Tarnow Offensive and the Great Retreat in 1915; the Brusilov Offensive (and subsequent Austro-German counterattacks) and the invasion of Romania in 1916; the Kerensky Offensives and the Capture of Riga in 1917; and Operation False Flag in 1918. All of these were major engagements, and involved a considerable amount of mobility and fluidity.

They also produced huge casualties. c. 2.1 million Russian casualties were suffered in 1915, half of which were prisoners, and a quarter of those casualties were suffered during Gorlice-Tarnow alone, c. 500 000 in the span of a month. In the course of the Brusilov Offensive and the Invasion of Romania, which dominated the Summer and early Fall of 1916 on the Eastern Front, c. 2 million Russian casualties were suffered, and perhaps half of those were killed.

The Austro-Hungarian army was gutted by the battles of 1914, losing 1 million men by Christmas. It was permanently crippled by the Brusilov Offensive, which inflicted c.750 000 to 1 million casualties between June 4th and August 15th, of which a quarter to a half of those were prisoners. The Kerensky offensives tore a gap in the Austrians lines, and managed to inflict c. 350 000 casualties before being checked and counter-attacked by the Germans and Austrians. Operation False Flag, which took place when the first round of negotiations at Brest-Litovsk failed, brought the Germans from the modern day border of Poland and Ukraine, almost to the Don River.

Sources: The Brusilov Offensive, Timothy Dowling; A Mad Catastrophe, Geoffrey Wawro; Collision of Empires, Prit Buttar; Ring of Steel, Alexander Watson; Great War: A Combat History of the First World War, Peter Hart